Hydrogen generator with improved demand control.
It has been known for some time that the reaction of alkali and/or alkali-earth metal hydrides is a convenient method of generating pure hydrogen for a variety of uses. One application which has been practiced for some time is the generation of hydrogen for filling of meteorological balloons. The calcium hydride water reaction has been relied upon for this application. Generators are commercially available. They provide instantaneous hydrogen with little if any control of the generation rate. A large surplus of water is applied to assure completion of the reaction.
More recently the interest in hydrogen generation from hydrides has increased because of the development of fuel cells, specifically Proton Exchange Membrane (PEM) fuel cells. The combination of these cells with hydrogen generators offers considerable advantages over primary and secondary batteries in terms of gravimetric and volumetric energy density and life cycle cost.
The use of these hydrogen generators with PEM fuel cells imposes increased need for demand/load responsive generator operation since intermediate hydrogen storage is for most purposes impractical. This need was recognized by the inventor and led to the development of a cartridge-type generator where calcium hydride particles were mixed with compressible foam sections and loaded into a metal tube. Water was admitted to this cartridge tube in a Kipp arrangement. A patent was granted U.S. Pat. No. 4,261,956--for this arrangement, which provided some improvement over prior art generator concepts.
A main disadvantage of this hydrogen generator configuration, however, was the large excess of water required to bring the hydride conversion to completion. This is a significant negative in the Unmanned Underwater Vehicle (UUV) application, where during deployment water may not penetrate the hull of the vehicle nor may hydrogen gas be released. It is a particularly stringent condition that the fuel cell product water is consumed for the generation of hydrogen by reaction with certain selected hydrides, such as calcium hydride or lithium hydride, which generate stoichiometric quantities of hydrogen for the amount of fuel cell product water generated. It is in these applications in particular where the quantity of water available to complete the reaction is limited indeed.
The requirement may not be as stringent in other applications. However, the need for satisfactory load response and efficient reactant utilization remains always an important requirement to meet performance and cost objectives.